1. Field of the Invention
The present invention generally relates to electron beam deflection systems and, more particularly, to arrangements for reduction of aberrations therein.
2. Description of the Prior Art
Electron beam deflection systems are known in many types of devices including cathode ray tubes used for display, as in televisions, oscilloscopes and computer displays. Electron beam deflection systems are also widely used in electron beam lithography systems, particularly for exposure of resists in the fabrication of masks for use in fabrication of integrated circuits and also direct writing for patterning of resists directly placed on substrates and other structures. As integrated circuit density has increased and pattern feature size correspondingly reduced, exposure of resists using an electron beam exposure system, sometimes referred to as an e-beam tool, in which an electron beam can be deflected over a surface with high speed and accuracy under computer control, has become highly advantageous and the exposure methodology of choice.
Electron beam deflection system typically operate to deflect the electrons of the beam by subjecting them to either an electrostatic or a magnetic field or both over a finite distance along the electron path through the deflection system. These fields achieve a change of velocity of the electrons in a direction normal to the undeflected path of the electron beam. Also, the extent of structure in the direction of the electron-optical axis of the system must also often be limited in order to perform different manipulations of the electron beam at different locations along the electron beam column. The magnetic or electrical fields which are imposed on the electron beam are designed to minimize errors at the target. However, a theoretically ideal geometry for an electron beam lithography system cannot be achieved in practice since any limitation of the extent of magnetic and electrical field generating structure causes fringe fields and degrades system performance from the ideal. Further, some imperfections arise from the construction of field producing structure regardless of the design and the actual location of the axis of such structure cannot be accurately determined other than through calibration and adjustment. A number of aberrations of the spot occurring over the deflection region are susceptible to correction using signals which are a function of the position coordinates where the beam is intended to land on the target. These types of correction are referred to as dynamic corrections. Some types of dynamic correction require specific magnetic or electric field producing structures.
Specifically, when the electron beam is deflected, the plane of best focus becomes curved and astigmatism is introduced. Field curvature causes changes in the size and current density profile across a projected pattern or spot. Astigmatism alters or distorts the cross-sectional shape of the beam as it impinges on the target.
Field curvature can be corrected by the use of a projection lens which alters the final focus of the beam at a point near a target. In a magnetic system, focus correction is generally done by means of a small coil within the projection lens. This also applies to electric fields. Magnetic fields are, however, more widely used because of their lesser chromatic aberrations and because the field generating elements can be placed outside the vacuum vessel; acting through the non-magnetic walls of the vessel. The examples in the following discussion therefore refer to magnetic fields and the associated field generating arrangements. Next to field curvature, astigmatism can also be corrected, to a degree, by subjecting the electron beam to a quadrupole magnetic field. In high precision e-beam tools and cathode ray tubes, correction of deflection is also normally applied by applying a dynamic correction based on the exposure spot address. specifically, for e-beam tools, a specific error is found for each of a plurality of locations on a target corresponding to particular deflection within a field of view of the deflection system and a specific correction value stored, typically in digital form in a look-up table. In use, correction values are retrieved from storage at each location of the beam and a correction derived by interpolation. To date, this has generally provided a satisfactory degree of correction of deflection. Dynamic focus correction has also been applied in the same fashion to correct for field curvature in high precision e-beam tools.
However, at higher component densities in integrated circuits, the tolerance for distortion and aberration becomes very much reduced. For pattern design rules having a particular minimum feature size, often referred to as a regime, the tolerance for geometrical errors of position and distortion of the electron beam are usually several orders of magnitude smaller. These errors can originate from several sources and the total cumulative allowable error is commonly referred to as an error budget. For example, in a regime having a minimum feature size of a fraction (e.g. about one-quarter) of a micron, the total error budget might be on the order of a few tens of nanometers.
Keeping the geometry of an electron beam deflection within this error budget is difficult. A particular complicating factor derives from the reciprocal effects in regard to deflection and astigmatism. Specifically, while astigmatism can be adequately corrected when the electron beam passes along the axis of symmetry of the quadrupole field, usually produced by electromagnets formed as four (or eight) coils and referred to as a stigmator, correction of astigmatism under an effectively asymmetrical magnetic field, as occurs when the axis of the electron beam varies from the axis of the stigmator, causes deflection of the beam. While this correction could be approximated by the dynamic correction of deflection, discussed above. Only approximate correction is possible for the simple reason that, if the electron beam axis varies with deflection, the alteration of deflection to compensate for positional error at a particular degree of deflection will result in overcorrection of the total deflection. Such approximate correction is not adequate to achieve the precision currently required to produce integrated circuits at the integration densities otherwise possible at the present state of the art.
Astigmatism requires correction by magnitude and azimuth. A single quadrupole, consisting of four coils, would therefore require rotation to mechanically correct for angular orientation (e.g. azimuth). This cannot be done with the speed required for dynamic correction. Two quadrupoles, comprising four coils each, are therefore arranged rotated by 45.degree. with respect to each other, preferably in the same plane. By selecting the magnitude, sign and ratio of the two exciting currents in the two quadrupoles forming the stigmator, magnitude and angular direction (azimuth) of the resulting quadrupole field can be controlled electrically.
In the prior art, only static correction of alignment of the stigmator coils and the electron beam axis could be done. This static correction required difficult, complex and extremely precise mechanical adjustment of the stigmator coils within the e-beam tool. Perhaps more importantly, the requirement that the stigmation coils be coaxial with the electron beam limited the locations the electron beam column where the stigmator could be placed. In contrast with the correction of field curvature which can be done close to the target and after deflection of the beam, the requirement for coaxial mutual positioning of the stigmator yoke and the electron beam has required placement of astigmatism correction in advance of all deflection stages in the electron optical column.
As further background, it is the practice when making mechanical adjustments of the positioning of the stigmation coils, to alter the current applied to the coils for correction of astigmatism and to observe any change in beam position at the target. This procedure essentially over- or under-corrects the astigmatism. However, since the deflection which will be caused is a function of both the astigmatism correction and the variance from coaxial positioning of the electron beam and the stigmator, the nature and magnitude of the error can be determined as a function of the resulting deflection.
Additionally, due to the precision required from the e-beam tool, it is customary to recalibrate the tool at frequent intervals of use. Since such recalibration involves non-productive "down-time" of expensive apparatus. it is economically desirable to carry out recalibration as rapidly as possible. Accordingly, some of the recalibration procedures have been automated under computer control, such as in the development of error data for dynamic correction of deflection, as discussed above. However, at the present state of the art, no technique for automation of adjustment of coaxial positioning of the stigmator and the electron beam has been available since the mechanical precision required is beyond the capability of servo systems.
At the present state of the art, it is known to shift the electron-optical axis of round (rotationally symmetric-magnetic lenses by electrical means, specifically by superimposing a deflection field generated by a deflection yoke meeting certain mathematical conditions regarding the shape of the field distribution along the axis. A Variable Axis Lens and its advanced development, the Variable Axis Immersion Lens (VAIL) are described in U.S. Pat. Nos. 4,544,846 and 4,859,856, assigned to the assignee of the present invention, and are used in practice.